This invention relates to expandable slotted tube which is used primarily in bore holes to form a lining which prevents collapse of the wall of the bore hole whilst allowing substantially unrestricted flow of fluids from the surrounding strata into the bore hole.
The use of such expandable slotted tube is described particularly in WO-A-93/25800. The expandable slotted tube (or xe2x80x9clinerxe2x80x9d as as it is referred to therein) is a product which can with advantage be used in all aspects of well life, including drilling, completion, well intervention and abandonment. Conventional oil field technology is based on inserting articles of fixed size into the well bore, and well construction is normally such that the size of the bore progressively reduces from the well head down to the reservoir to be tapped. This can lead to an unworkably small bore size at the required depth, leading to compromises in well operability or, in an extreme case, resulting in failure to reach the reservoir at a bore size sufficient to enable the reserves to be exploited.
The use of expandable slotted tube provides flexibility to cope with unexpected problems during both drilling and completion of the well and also during the life of the well as compared with conventional well construction techniques.
In particular, expandable slotted tube can be designed to fit most well bore sizes, whilst being made small enough to pass through pre-existing restrictions in the bore. This is largely as a result of the phenomenon known as xe2x80x9csurplus expansionxe2x80x9d which occurs when the expandable slotted tube is expanded by drawing an expansion mandrel through it, that is to say the expanded internal diameter of the tube increases to a value greater than that of the maximum outside diameter of the expansion mandrel by a factor which is dependent at least in part on the mandrel design.
The expandable slotted tube as described in WO-A-93/25800 can be expanded up to 200% of its original diameter and can in theory be made in any size or length.
However, there are problems in the manufacture of such expandable slotted tube, and in particular in ensuring uniform radial expansion.
In practice, the tube in which the slots have to be formed will, in general, not be accurately cylindrical and straight over the whole of its length. The longitudinal axis of the tube may deviate from a straight line over all or part of its length, and the circumferential shape of the tube may deviate from accurately circular over part or all of its length, due to limitations inherent in the manufacturing processes conventionally employed. As a result, if a pre-determined and invariable pattern of slots is cut into such a tube such defects will be magnified by its subsequent radial expansion and may then exceed acceptable tolerances.
Moreover, in practice, such tubes must be joined together to form a liner of the required overall working length corresponding to the depth of the borehole. Accordingly, appropriate coupling fittings must be provided at opposite ends of each tube to form a composite fabrication comprising a tubular element of appropriate length and annular elements forming the coupling fittings at the ends thereof.
Further, each length of such fabricated tube may include one or more additional fittings intermediate its ends, known as xe2x80x9clanding ringsxe2x80x9d, which are also formed as separate components, the tube then comprising two (or more) of said tubular elements connected together in end-wise relation by an annular element formed as a landing ring, and the two annular elements forming the end coupling fittings.
Thus, typically, a length of tube as fabricated may comprise at least five elements, namely a female end coupling at one end, a first tubular element, a landing ring, a second tubular element, and a male end coupling at the other end. The couplings and landing ring are fittings that are normally formed to a high degree of accuracy by machining operations, and these fittings necessarily have different characteristics from the tubular elements. However, the individual components are secured together in endwise relation by means such as welding, and the fabrication of the complete tube in this way inevitably introduces variability in a number of respects from one tube to another, even though nominally identical.
In particular, the overall length of the fabricated tube, which is typically in excess of 10 meters, may vary by several millimeters as a result of the accumulated tolerances on the lengths of the individual tubular elements and the thickness of each weld. Additionally, as a result of potential distortions within each tubular element, the couplings at opposite ends of the tube will in general not be accurately in register with one another circumferentially, that is to say one may be angularly offset relative to the other about the centreline of the tube. This is of significance since the couplings are normally formed with axially interfitting formations, and the slots which are formed in the tube must be correctly orientated relative to such interfitting formations at both ends of the tube.
Additionally, the accuracy, with respect to location and length, of the slots which extend across locally thickened portions of the tube, for example in the landing rings and end couplings, is critical.
Accordingly the object of the invention is to facilitate the manufacture of expanded slotted tube in a manner which satisfactorily takes such factors into account.
According to the invention we provide a method of manufacturing an expandable slotted tube of a kind which is fabricated from at least one tubular element formed by extrusion or similar processes and at least two annular elements formed by machining or like processes and connected to the opposite ends of the tubular element by means of butt joints formed by welding or like processes, wherein axially extending slots are formed at a uniform angular spacing around the periphery of the tube fabricated from said elements and in longitudinally overlapping relation throughout the entire length of said fabricated tube, including said tubular element and said annular elements, the method being characterised by cutting in said annular elements a pattern of overlapping positionally fixed slots in accordance with fixed and invariable parameters with regard to their longitudinal position and length which are pre-determined relative to the dimensions of said annular elements, and cutting in said tubular element a pattern of positionally variable slots that is variable with regard to at least the parameters of longitudinal position and length of the slots in accordance with measured parameters of the fabricated tube so as to achieve the required overlap between said positionally fixed slots and said positionally variable slots.
The method may include the steps of
a) establishing longitudinal datum points for each of said annular elements,
b) calculating, by reference to said longitudinal datum points and by reference to the pre-determined parameters of longitudinal position and axial length of said positionally fixed slots, the nominal longitudinal position and axial length of said positionally variable slots required to be cut in said tubular element so as to ensure that those positionally variable slots that extend across a joint between said annular elements and said tubular element are longitudinally centred on the joint across which they extend, and the length of each overlap between said positionally variable slots, including said joint-crossing slots, is uniform, and
c) cutting said slots in accordance with said pre-determined and said calculated parameters.
The method also preferably includes the further step of monitoring at the cutting site the position of the local centre-line of the tube and the radial distance from said centre-line to the outer surface of the tube, and recalculating the nominal parameters of the positionally variable slots so as to correct substantially uniformly over the length of the tubular element for any local deviations in the position of the centre-line and/or in circularity of the tubular element.
The method is preferably carried out by commencing the cutting of said slots from one end of the fabricated tube to form said slots progressively along the entire length of the tube.
The longitudinal datum points may include a first point corresponding to a first end of the tube as defined by an end face of the annular element at that end of the tube and a second point corresponding to the other end of the tube as defined by an end face of the annular element at that end of the tube, plus third and fourth points corresponding respectively to the joints between the tubular element and the annular elements at those ends of the tube. It will be appreciated that the annular elements at those ends of the tube may comprise couplings for the releasable connection of two similar tubes, in particular male and female couplings which are adapted to fit at least partially one inside the other and accordingly are in part of greater external diameter than the tubular element to which they are joined.
Normally, the fabricated tube will also include a further such annular element which is disposed intermediate the ends of the tube, which then comprises two of said tubular elements, such intermediate annular element being connected thereto by butt joints. Such intermediate annular element may be formed with an external flange for the purpose of lifting the fabricated tube, and therefore also be of greater external diameter than the tubular element to which it is joined. In such a case the datum point for this intermediate annular element may be afforded by a radial face of the flange. The method previously described may accordingly be applied to the manufacture of slotted tube of the kind which the tubular element has a further tubular element connected thereto by means of one of said annular elements and said further tubular element has a further annular element connected at the end thereof, and the annular element which is disposed intermediate the tubular elements is formed with an external flange, by establishing a further datum point corresponding to a radial face of the flange on said intermediate annular element.
In accordance with a preferred feature of the invention an angular datum is also established relative to the centre-line of the fabricated tube at one end thereof, and cutting is commenced at the other end so that any angular off-set which is required at the latter end to align the slots correctly with respect to the annular element at that end is determined relative to said angular datum and is included in the re-calculation of the parameters of said positionally variable slots. The angular position of the positionally fixed slots may also be included in this re-calculation.
According to a further aspect the invention also resides in expandable slotted tube as manufactured by the above method.
Further, the invention also resides in a machine for use in the manufacture by the above method of expandable slotted tube of a kind which is fabricated from at least one tubular element formed by extrusion or similar processes and at least two annular elements formed by machining or like processes and connected to the opposite ends of the tubular element by means of butt joints formed by welding or like processes, wherein axially extending slots are formed at a uniform angular spacing around the periphery of the tube fabricated from said elements and in longitudinally overlapping relation throughout the entire length thereof, including said tubular element and said annular elements, and comprising
a) means for establishing longitudinal datum points for each of said annular elements,
b) means for calculating, by reference to said longitudinal datum points and to the pre-determined parameters of longitudinal position and axial length of said positionally fixed slots, the nominal longitudinal position and axial length of said positionally variable slots required to be cut in said tubular element so as to ensure that those positionally variable slots (hereinafter called xe2x80x9cjoint-crossingxe2x80x9d slots) that extend across a joint between said annular elements and said tubular element are longitudinally centred on the joint across which they extend, and the length of each overlap between said positionally variable slots, including said joint-crossing slots, is uniform, and
c) means for cutting said slots in accordance with said pre-determined and calculated parameters.
The cutting means is preferably arranged to cut said slots progressively from one end of the fabricated tube to form said slots sequentially along the entire length of the tube.
The machine also preferably includes means for monitoring the position of the local centre-line of the tube and the radial distance of the outer surface of the tube at the cutting site from said centre-line, and recalculating the nominal parameters of the positionally variable slots so as to correct substantially uniformly over the length of the tubular element for any local deviations in the position of the centre-line and/or in circularity of the tubular element as the cutting progresses along the tubular element.
Such machine preferably also includes means for establishing an angular datum relative to the centre-line of the fabricated tube at one end thereof, means for providing an angular off-set relative to said angular datum of said cutting means at the other end of the tube to align the slots correctly with respect to the annular element at that end, and means for including said angular off-set in the re-calculation of the parameters of said positionally variable slots as cutting progresses.
The machine may comprise a plurality of supports adapted to hold a fabricated tube for cutting, an indexing rotator connectable to said tube for rotating said tube about its longitudinal axis in step-wise increments, a longitudinal guide defining a first longitudinal axis parallel to that of said tube when mounted on said supports, a main carriage mounted on said longitudinal guide for movement along said first longitudinal axis and along the entire length of said tube, a sub-carriage mounted on said main carriage for movement relative thereto on two transverse axes relative to said longitudinal axis of the tube so that said sub-carriage can thereby be centred relative to said longitudinal axis of the tube and for movement on a second longitudinal axis which is parallel to said first longitudinal axis, at least one slot cutter mounted on said sub-carriage for movement radially relative to said longitudinal axis of the tube, and a control system operative to move said main carriage through a succession of working positions along the longitudinal guide, in each of which positions the sub-carriage is centred relative to said longitudinal axis of the tube and a sequence of slot cutting operations is performed by moving said sub-carriage along said second longitudinal axis while said slot cutter is operated to cut a slot of pre-determined length, rotationally indexing said tube and returning said sub-carriage to its starting position on said second longitudinal axis and repeating the cutting operations until all the required slots are formed at each of said working positions.
Preferably, the machine includes a plurality, e.g. four, of said slot cutters which are arranged at equal spacings around longitudinal axis of the tube so that a plurality of slots can be cut simultaneously each time the tube is indexed. The slot cutters are preferably abrasive water-jet cutters, although other types of cutter could be employed.